1. Field of the Invention
The present invention relates to a thickness measurement mechanism and a coating layer forming apparatus using the same, for forming a coating layer in a prescribed thickness range on an elongate base material while it is fed.
2. Description of the Background Art
A feeding mechanism provided with a treatment base station (hereinafter, simply also referred to as a base station) for forming a coating layer on a base material surface continuously at a prescribed speed by using physical or chemical means while conductive elongate base materials of various cross-sectional shapes are fed is used for forming a layer to a thickness in a prescribed range over the entire length of the base materials. Here, examples of the base material include various materials ranging from a soft material such as copper, aluminum, or resin containing metal filler to a hard material such as tungsten or steel. In such a feeding mechanism, it is necessary at least at the base station to keep constant the speed of passage of the base material. Therefore, it is necessary to apply appropriate tension to the base material before and after it passes through the base station, and to feed the base material along the same path over the entire length. In the base station, as the tension is applied to the fed base material, friction due to sliding contact between the base material and a feeding member or a member for providing a function is caused, and frictional force greater than that between the base material and the feeding member on a supply side or on a recovery side is applied to the base material.
Among others, it is difficult to adjust the balance of feeding tension when a thin layer is to be formed with high dimensional accuracy on the surface of the elongate base material in the form of a tape or fiber. A so-called winding type thin film forming apparatus, in which an elongate strip-shaped base material (hereinafter simply also referred to as a tape) is fed into a thin film forming chamber and the film is continuously formed on the base material, is widely used for manufacturing various tapes including tapes for magnetic recording, printers and wrapping. Normally, the base material is fed using a feeding member such as rollers provided at a portion for supply to the base station and a portion for recovery from the base station, while the base material is kept in touch with constant tension on the surface of the feeding member. Here, conventionally, there have been problems of the degree of close contact between the tape and the feeding member surface, variation in thickness in the formed layer resulting from fluctuation in feeding speed and damage to the layer surface, and measures to avoid such problems have been considered.
Such measures include (1) those related to a feeding mechanism with attention being paid to a structure or arrangement of a feeding member for supplying or recovering the base material, and (2) those for continuously sensing thickness information calculated in accordance with a surface state or a physical property of a layer and feeding back the information to the feeding member or a manufacturing parameter system affecting the thickness.
For example, Japanese Patent Laying-Open Nos. 62-247073, 61-264514, and 61-278032 propose exemplary means as defined in (1). These publications all propose approaches to suppress variation in the degree of close contact of the tape to the surface of the feeding member such as rollers above. Japanese Patent Laying-Open No. 62-247073 proposes a feeding mechanism where a speed-variable sub-roller is provided before and after the base station, Japanese Patent Laying-Open No. 61-264514 proposes a feeding mechanism where a dancer roller is provided before and after the base station, and Japanese Patent Laying-Open No. 61-278032 proposes a feeding mechanism where a plurality of guide roller pairs are provided on the recovery side. These means, however, cannot be free from variation in thickness of the layer or occurrence of damage, as the speed of the base material is varied due to change over time in an outer diameter of a feeding portion on the supply side and on the recovery side.
A large number of means as defined in (2) utilize relation between a capacitance and a thickness. For example, Japanese Patent No. 2922376 and Japanese Patent Laying-Open Nos. 7-280503 and 2004-12435 present exemplary means. Japanese Patent No. 2922376 discloses means for measuring a thickness using a capacitance and an electric resistance of a sheet. With this means, however, in particular, when a layer has a specific resistance value (electric resistance specific to a material) not smaller than that of a semiconductor, in particular, not smaller than 100 kΩ·cm, dependency of the layer thickness on the electric resistance is lowered, and accuracy in measurement is lowered. Japanese Patent Laying-Open No. 7-280503 discloses means for shaping a contact electrode sensing a capacitance in a rotatable cylinder. With this means, however, in particular, it is difficult for rotation of the electrode to follow unevenness of the layer surface, and accuracy in measurement is lowered. According to Japanese Patent Laying-Open No. 2004-12435, initially, a position of a base material surface or a formed layer surface is measured using a displacement gauge. Then, the measured position and a sensor are set to be equidistant, and the formed layer and the sensor are set in a non-contact state. Then, a capacitance between the base material and the sensor is measured. Thereafter, a thickness is calculated based on correlation between the capacitance and the thickness of the formed layer that have been found in advance. With this means, however, particularly if uneven base material surface is moving, the displacement gauge cannot immediately follow the movement. Accordingly, time lag from sensing of a distance by the sensor is caused, which leads to lower accuracy in measurement. Therefore, if the base material is out of a limited range of base material conditions, variation in thickness of the layer or occurrence of damage is unavoidable.